Bile flow depends upon both the transport of bile acids by carrier-mediated mechanisms and the active secretion of sodium, probably by (Na-K)ATPase, the in vitro equivalent of the sodium pump. We have examined the effect of pharmacological agents (phenobarbital and ethinyl estradiol) and hormones (steroids and thyroid) on quantitation of bile acid receptors, (Na-K)ATPase and lipid content of liver surface membranes. These studies demonstrate that alteration of (Na-K)ATPase activity by either drugs or hormones is associated with correlative changes in bile flow. However, the mechanism for changes in activity differ; that is, phenobarbital and glucocorticoids apparently increase synthesis of new enzyme units. In contrast, ethinyl estradiol changes the lipid composition and membrane viscosity apparently decreasing its activity by altering the environment of the transport proteins. Although ethinyl estradiol reduces bile acid maximum transport capacity, the affinity and number of bile acid receptors was unaltered by treatment. However, correction of the abnormal lipid environment also corrected bile flow and bile acid transport to control. These studies demonstrate that pharmacological and hormonal agents selectively alter liver surface membrane proteins and/or lipids which in turn may regulate bile flow.